1. Field of the Invention
The present invention relates to a pressing apparatus, and more particularly, to a pressing apparatus having a plurality of rollers forming a chamber.
2. Description of the Related Art
For many years attempts have been made to use external air pressure to force water out of a paper web. Rather than compress a sheet at a press nip to the point where hydraulic pressure drives water out, as is the case in normal wet pressing, it was reasoned that more water could be removed, and sheet bulk could be maintained, if air pressure could be applied to supplement roller nip generated hydraulic pressures. One such attempt involves providing a multi-roller structure forming a closed chamber, wherein air is circulated through the chamber to convect moisture out of the paper web.
Providing efficient sealing of a multi-roller chamber can be problematic. It is known to form a roller assembly wherein rubber rollers are positioned to interact with solid surface rollers. One potential problem in trying to seal such a chamber is that considerable loading to the roller structure may required to maintain the seal between the rollers. Accordingly, a robust frame is required to confine the roller structure. Another potential problem in trying to seal such a chamber is that any cuts into the rubber surface would tend to render the entire roller unusable.
Also, it has been recognized that conventional wet pressing methods are very inefficient in that only a small portion of a roller""s circumference is used for processing the paper web. To overcome this limitation, some attempts have been made to adapt a solid impermeable band to form an extended nip for pressing the paper web to de-water the paper web. One problem with such an approach, however, is that the impermeable band prevents the flow of a drying fluid, such as air, through the paper web.
Accordingly, a need exists for an improved fabric which provides enhanced de-watering of a continuous web and provides efficient sealing of a chamber at the roller nips.
The present invention provides enhanced de-watering of a continuous web, such as paper, and provides efficient sealing of a chamber at the roller nips, in a pressing apparatus.
One aspect of the invention is a unitary membrane for use in a pressing apparatus. The pressing apparatus includes a pair of longitudinal edge portions and a semipermeable portion having a plurality of intercommunicating pores. The semipermeable portion is positioned between the pair of longitudinal edge portions. The unitary membrane includes a formed fabric and has a thickness less than about 0.1 inches. The semipermeable portion has a permeability greater than zero and less than about five CFM (cubic feet per minute) per square foot as measured by TAPPI test method TIP 0404-20.
In some embodiments, the pair of longitudinal edge portions are tapered such that a cross-section of said unitary membrane has a trapezoidal shape. Also, preferably, the pair of longitudinal edge portions are impermeable.
Another aspect of the invention is a method of making the unitary membrane and includes the steps of providing a carrier fabric which is very permeable and forming a plurality of intercommunicating pores in the carrier fabric.
An advantage of the present invention when used in a pressing apparatus having a pressurized chamber formed by a plurality of rollers is that the invention can effect both a predetermined fluid flow through and a mechanical pressing force on a continuous web, such as a paper web, to promote enhanced de-watering of the continuous web.
Another advantage of the invention when used in a pressing apparatus is that the invention aids in sealing a pressurized chamber formed by a plurality of rollers at two or more roller nips.